I think I have a working strategy. Step 1: >!Play as X. Step 2: >!Before your first turn, ask this: "If my opponent wins, will it be along a horizontal line?" Step 3: >!Regardless of the answer, play in the centre. Step 4: >!After the opponent's first move, play in a corner. This leaves 4 possible board states (removing reflections and rotations). Lines where the O player can win are marked in red. [![the four possible board states after one turn from both players][2]][2] >!Now note that regardless of which possibility is true, the O player has only two lines they can win on. Additionally, one of these has become unplayable due to the question. Regardless of what O plays here, X always has a move which blocks O's final possible win line. [2]: https://i.sstatic.net/zkmByX5n.png